Properties of Y-type zeolites with various silicon/aluminium ratios obtained by dealumination with silicon tetrachloride. Distribution of aluminium and hydroxyl groups and interaction with ethanol

Abstract

Y-type zeolites with Si/Al ratios of 3.4–20, prepared by the dealumination of NaY (Si/Al = 2.5) with SiCl₄, have been characterized using X-ray diffraction analysis, sorption-capacity data and by obtaining infrared spectra of the framework, OH groups and adsorbed pyridine. Dealuminated zeolites exhibit good crystallinity and some aluminium released from the lattice remains in extralattice positions in the zeolite cavities or in an amorphous phase to provide strong electron-acceptor sites. The zeolites were obtained directly in the H form; further exchange with NH⁺₄ affected their proton-donor properties only slightly. In addition to weakly acidic or non-acidic hydroxyls (bands at 3740 and 3620 cm^–1), strongly acidic framework hydroxyls were present (bands at 3630 and 3560 cm^–1) in amounts dependent on the amount and distribution of aluminium. An analysis of substances released during temperature-programmed desorption from zeolites with preadsorbed ethanol revealed the influence of dealumination on the catalytic activity: the maximum of evolved ethylene as a main product was reached on moderately dealuminated zeolites (Si/Al = 3.4–6) with an appreciable number of both strong electron-acceptor and strong proton-donor sites.